In the United States, approximately half of all breast cancer patients are diagnosed with estrogen receptor- positive (ER+) lymph node negative (LN-) primary tumors. Two adjuvant systemic treatment options exist for such patients and include hormonal therapy alone or hormonal therapy and chemotherapy. Choosing the appropriate adjuvant regimen for ER+LN- breast cancer patients remains a significant challenge. We have developed a novel real-time PCR assay, HOXB13:IL17BR-MGI, which is a robust, independent prognostic and predictive biomarker of both tamoxifen and aromatase inhibitor hormonal therapy. This biomarker identifies a subgroup of early stage ER+ breast cancer patients with very poor outcome despite endocrine therapy. The long-term goals of this project are: 1) to assess the chemotherapy treatment-predictive utility of the HOXB13:IL17BR-MGI biomarker in ER-positive lymph node negative breast cancer patients; 2) to compare the prognostic performance of the HOXB13:IL17BR-MGI biomarker to OncotypeDx; and 3) to assess the prognostic and predictive performance of the HOXB13:IL17BR-MGI biomarker in ER-positive breast cancer patients treated with adjuvant aromatase inhibitor therapy. Specifically, in Aim 1, we will assess the ability of the HOXB13:IL17BR-MGI biomarker to predict chemotherapy benefit in tumor samples from the International Breast Cancer Study Group Trial IX breast cancer cohort. In Aim 2, we will compare the prognostic performance of the HOXB13:IL17BR-MGI biomarker with that of the Oncotype DX assay in a cohort of 800 patients who have undergone Oncotype DX testing, and we will assess whether the HOXB13:IL17BR-MGI biomarker provides additional prognostic information to the OncoDX-RS assay. Finally, in Aim 3, we propose to assess prognostic and predictive biomarker performance in samples from the NCIC CTG- and NCI-sponsored MA.17 adjuvant aromatase inhibitor therapy trial. Accomplishment of these goals will improve the identification of ER+LN- breast cancer patients who will benefit from hormonal therapy alone and those who will benefit from the addition of adjuvant chemotherapy, and may provide for novel therapeutic strategies for women with tamoxifen-resistant disease.